The removal of sulfur from gasoline fuel demands attention worldwide, not only because of the need to reduce atmospheric pollution by sulfur oxides, but also because of the need to make ultra-low sulfur fuels for hydrocarbon fuel processors used in fuel cell applications. EPA regulations put forward in 2001 require that gasoline sulfur content must be ≦30 ppmw, and highway diesel sulfur content should be ≦15 ppmw in 2009.
The common types of sulfur compounds in various distillate fuel fractions include sulfides, disulfides, thiols, thiophenes, benzothiophenes, methyl-benzothiophenes, dibenzothiophenes, and methyl-substituted dibenzothiophenes. The presence of sulfur compounds in commercial fuels is highly undesirable. These compounds are corrosive to metals, poison catalysts in hydrocarbon fuel processors, and they contaminate the environment in the form of sulfur oxides emitted in engine exhaust.
Currently, the extent of petroleum feedstock desulfurization depends on the catalytic hydrodesulfurization process (HDS), where the sulfur compounds lose sulfur by hydrogenation reactions, giving off H2S as one of the treatable products. Hydrotreating is a commercially proven and simple refining process, and refineries with hydrotreaters produce deeply desulfurized gas oil on straight-run distillates by modifying catalysts and operating conditions. However, greater challenges are expected for desulfurizing distillate streams such as Light Cycle Oil (LCO), requiring either substantial revamps to equipment or construction of new units. Specifically, hydrotreating LCO requires a higher reactor pressure, as well as an increased hydrogen rate and purity. Furthermore, distillates from Fluid Catalytic Cracking (FCC) operations contain higher concentrations of compounds with aromatic rings, which make deep desulfurization more difficult. For these reasons, new technology developments are needed for the ultra-deep desulfurization of these feedstocks.
In order to reduce the cost of deep-desulfurization, several new technologies have been introduced in the experimental stages. Among them, sulfur adsorption, sulfur oxidation and biodesulfurization seem to be quite promising. The major advantages of these new technologies include lower costs, lower processing temperatures and pressures, reduced emissions of gaseous pollutants and carbon dioxide, and no hydrogen requirement. In general, the sulfur adsorption processes use specific adsorbents that interact with the sulfur-containing compounds to separate them selectively from the hydrocarbon mixtures. This technology seems particularly favorable for gasoline desulfurization because the process does not modify the hydrocarbon components, thereby avoiding any loss in octane rating.
In commercial diesel, the major sulfur compounds are thiophene, benzothiophene, dibenzothiophene, and their alkyl derivatives. This fact indicates that the reactivities of alkyl-substituted benzothiophenes (BT) and dibenzothiophenes (DBT) are much lower during catalytic hydrotreating than those of other sulfur-containing molecules. Kabe et al. reported that although the alkyl group substitutions of DBT do not inhibit the adsorption of DBT's on catalysts via π-electrons in the aromatic rings, the C—S bond cleavage of adsorbed DBT's is disturbed by steric hinderance of the alkyl group(s). Kabe, T.; Ishihara, A.; Zhang, Q. Deep desulfurization of light oil. Part 2: hydrodesulfurization of dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Appl. Catal. A 1993, 97, L1-L9. Consequently, in the ultra-deep desulfurization process, the removal of these substituted DBT's is of greatest interest for refineries.
Because DBT's are electron rich, they form charge transfer complexes (CTC) with suitable electron acceptors. For this reason, reversible complexation of DBT's by π-acceptors can be used as a separation strategy to recover DBT's. One technical challenge to overcome in order to use reversible complexation as the strategy for DBT removal from gasoils is that gasoils contain numerous other aromatic compounds that also can donate electrons to form CTC's with the acceptor compound. For this reason, the acceptor compound (or, more generally, the separation agent) needs to be selective toward the DBT's. To tackle this critical need, we have previously (i) prepared and tested a TAPA functionalized adsorbent that incorporates π-acceptor groups known to be efficient and selective for binding DBT's; (ii) addressed that this adsorbent should maintain capacity in the presence of significant volume percentages of aromatics; and (iii) addressed that this adsorbent is regenerable (i.e. complexation is reversible), as fully described in commonly assigned, pending U.S. patent application Ser. No. 12/134,311, the entire disclosure of which is hereby incorporated by reference in its entirety. We now address three issues pertaining to the use of TAPA functionalized adsorbents: (i) adsorption of 4,6-DMDBT in the presence of competing aromatics, (ii) co-adsorption of 4,6-DMDBT and dibenzothiophene from model diesel, and (iii) solvent regeneration of adsorbents with a toluene regenerant.